Fix: liblttng-ctl: config and mi strings inadvertantly exported

abidiff reports that a number of configuration and MI string symbols
are exported by liblttng-ctl:

'const char* const config_event_type_userspace_probe'    {config_event_type_userspace_probe}
'const char* const mi_lttng_element_command_disable_rotation'    {mi_lttng_element_command_disable_rotation}
'const char* const mi_lttng_element_command_enable_rotation'    {mi_lttng_element_command_enable_rotation}
'const char* const mi_lttng_element_command_rotate'    {mi_lttng_element_command_rotate}

Those strings are marked as LTTNG_HIDDEN.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>